Multires Normal Baking Leaving Blank Image
I solved this after messing around for about 45 minutes - posted anyway in case someone else is as boneheaded.  The multires needs to be set to "Render".  Once I did this, it worked fine.  

EDIT: To clarify, the multires modifier "render button" (the button that determines whether or not the modifier is used during renders) must be "on" when baking the normals.